This invention relates generally to carbon brushes or the like, and more specifically relates to the connection of an electrically conductive flexible lead cable to such brushes.
The commutator contact brushes utilized in electrical rotating machinery commonly comprise a block or body of carbon, graphite, metal graphite, or the like (herein generically termed "carbon"), which body is provided with flexible copper conductors or leads, usually of the stranded copper wire cable or pigtail type. These leads or "shunt connections" are secured to the carbon block by embedment. In particular, a hole of somewhat larger diameter than the shunt, is drilled into the top part of the brush, and the sides of the hole scored and rifled. The end of the shunt is then fitted into the hole to the bottom, after which a free-flowing conductive tamping powder is fed into the hole in the space surrounding the shunt and tamped to anchor the shunt to the brush. If desired, several drops of a binder or sealant, such as a phenolic resin dissolved in a suitable agent such as methanol, are applied atop the shunt connection to saturate the tamping powder in this zone, and to help seal out moisture. The sealant may then be heat cured, e.g. by application of infra-red radiation.
Further details regarding carbon brushes, and the prior art techniques for effecting connections of the type discussed, may be found in a number of references, notably including McCafferty, U.S. Pat. Nos. 3,510,710 and 3,666,688. A typical composition of the type set forth in the McCafferty patents may include a mixture of 95% pelletizable graphite powder, and 5% phenolic resin powder, which mixture is blended, compacted, and milled to an appropriate size prior to use. These compositions have found considerable acceptance as tamping compositions for use in the aforementioned environments.
Notwithstanding the generally high degree of utility of the tamping materials above discussed, it has been found that in certain high performance environments difficulties can occur. The aforementioned carbon brushes e.g. when utilized for traction motors on high horsepower locomotives or the like, are designed to carry a high electrical current, typically of the order of 125 amps or so. Thus in a typical bias cut design utilized in such environments all of the current passes through two such shunts or copper cables; these cables are of comparatively heavy construction, and may be rated for at least 60 amps. In such traction motor environments, shunt failures occur with a relatively unacceptable frequency, wherein it appears the tamping powder is jarred from the hole by vibration, after which the shunt cable becomes loose, or comes out of the hole entirely. If a sufficient number of shunts pull loose in this manner, the locomotive ground relay may trip, thus preventing the locomotive from traveling under its own power and pulling its share of railroad cars. Shunt failures of the type mentioned, have in the past frequently been attributed to the lack of sufficient tamping force, or to failure to utilize a sufficient number of tamping strokes during the tamping operation. Prior corrective action was therefore aimed at maximizing the effectiveness of the tamping operations. Despite these measures, shunt failures have not been markedly eliminated or prevented.
Continued investigation appears to indicate that shunt failures of the above type are frequently accompanied by evidence of overheating. In addition, the number of brush failures increases as accumulated locomotive mileage increases. These observations suggest that a likely failure mechanism is in fact thermal degradation of the resin in the tamping powder, due to long-term thermal cycling, e.g. up to 350.degree. F and higher, in the presence of mechanical vibration.
In accordance with the foregoing, it may be regarded as an object of the present invention to provide an improved tamping powder composition which, when used in effecting connections to carbon brushes, provides a durable shunt connection, able to resist progressive degradation and failure in high current, high temperature environments, with consequent low probability of service failure.
It is a further object of the invention to provide a carbon brush and method for preparing same, wherein the tamped interconnection between shunt cable and brush body is so effected as to provide a durable, high strength and low resistance shunt connection; which connection further displays very low probability of service failure, especially in high current, high temperature environments.